1. Field of the Invention
The present invention relates to oriented grain silicon steel that has very low core loss, and to a method of manufacturing same.
2. Description of the Prior Art
Grain oriented electrical steel sheet is extensively used as a material for magnetic cores. There is a particular demand for low core loss steel in order to reduce energy loss. An effective way of reducing core loss is to impart tension to steel sheet. The Tension can be effectively imparted to steel sheet by high temperature formation of a coating constituted by a material that has a smaller coefficient of thermal expansion than that of the steel sheet. During the finish annealing step, a layer in which the main component is forsterite is formed by reaction of surface oxides on the steel with the annealing separator. This coating gives a high degree of tension to the steel sheet and is effective for reducing core loss.
A highly effective method of imparting tension to steel sheet and thereby reducing the core loss is the method disclosed by JP-A-48-39338, whereby an insulating coating is formed by baking on a coating liquid in which the main components are a colloidal silica and aluminum phosphate. Therefore forming an insulating coating that imparts tension, after retaining the film formed in the finish annealing process, has become the normal method of producing grain oriented electrical steel sheet.
On the other hand, recently it has become clear that disordered interface structure between the forsterite-based layer and the base metal reduces the tension effect on core loss. Thus, JP-A-49-96920 and JP-A-04-131326, for example, disclose a process for reducing core loss by removing the forsterite layer formed by the finish annealing process and giving the steel surface a mirror finish before applying a tension coating.
When the forsterite layer is removed, or when the formation of such a layer is intentionally avoided, it becomes necessary to ensure the requisite tension with just the insulating layer. Thus, the insulating layer has to be made thick enough for this purpose, but the prior art does not clarify just what the necessary thickness is. In addition, a coating liquid that is applied over a film that is mainly constituted of forsterite has quite good adhesion. But when the forsterite layer is removed or when the formation thereof is intentionally avoided in the finish annealing, the coating has insufficient adhesion. Therefore it is necessary to improve the adhesion of the insulating coating to the steel sheet. For the above reasons, the effect of the mirror surface finishing according to the prior art was not enough to provide a sufficient reduction in core loss properties.
JP-A-60-131976 discloses forming method of insulating coating on the mirror finished steel sheet. Namely, by annealing steel sheet in a relatively weak oxidizing atmosphere in order to form a SiO.sub.2 oxide layer that is about 0.05 to 0.5 .mu.m thick, and then forming the insulating coating. According to the disclosure, the SiO.sub.2 oxide layer is comprised of SiO.sub.2 precipitates dispersed in the steel. However, as described below, an insulating coating has poor adhesion when the oxide layer is comprised by the dispersed SiO.sub.2 precipitates in the steel, a so-called internal oxide layer. As such, defining the SiO.sub.2 content may not be enough to achieve a coating having good adhesion.